The disclosure concerns a method and a device to regulate a property of a print image printed on a substrate material, in which method and device a measurement value is determined with the aid of an optical sensor. The measurement value is compared with a desired value. The inking of the substrate material is controlled depending on the result of this comparison.
The method and the device can in particular be used to regulate the optical density of the print image in electrographic color printers operating in print periods. In a print period, the individual color separations required for the print image are applied successively and overlapping onto a transfer element with the aid of developer units, and are transferred from the transfer element onto the substrate material after all color separations required for the print image to be generated have been applied onto the transfer element. For this the substrate material (designed in the form of a printing substrate web) must be periodically halted and accelerated again, corresponding to the print period. For example, the toner applied onto the substrate material is fixed onto the substrate material with the aid of a fixing unit operating with radiant heat. In order to prevent damage to the substrate material, cover units—blinds, for example—are driven between the heating elements of the fixing unit and the substrate material as soon as the substrate material is stopped in order to protect the substrate material arranged within the fixing unit from too much thermal radiation during a standstill. If the substrate material is driven again, the cover units are retracted so that the additional print image is fixed on the substrate material. This thus leads to fluctuations of the heat acting on the substrate material in the fixing unit. Due to the high heat sensitivity of the toner, these heat fluctuations produce fluctuations in the optical density and/or the gloss of the print image printed on the substrate material.
From U.S. Pat. No. 6,081,677 A, a method is known to optimize the semitone presentation in electrophotographic printing and copying devices in which a bias potential and/or a toner concentration is varied depending on an integral optical density (determined over the surface) of a raster toner mark on a photoconductor. The bias potential serves to adjust an auxiliary transmission voltage to transfer toner particles onto the photoconductor. What is disadvantageous is that no periodic fluctuations are detected; rather, only a general regulation takes place of the toner quantity to be applied during the collection period. The same toner quantity is applied during the entire collection period.
A method and an arrangement to adjust the dot size of print images generated with the aid of an electrographic printing or copying system are known from the document WO 2008/071741 A1. A measure of the area of a toner mark that is actually inked with toner particles is hereby determined as a real value and is compared with a desired value. Depending on the result of this comparison, an electrical field (BIAS potential) is adjusted to transfer toner particles onto the regions of a latent raster image that are to be inked. It is hereby disadvantageous that again only influencing factors that occur before or at the application of the toner image onto the photoconductor are taken into account.